بِسم الله الحمن الرحيم
From the moment that RasoolAllah ﷺ began to propagate Islam, the Makkan unbelievers opposed him intensely. Despite this, after 10 years they realized that Islam was just growing and most of the Muslims had emigrated to Madinah Munawwarah. Some of them met, and the meeting was personally attended by Iblis who posed as the Najdi Shaykh. They resoluted to assassinating RasoolAllah ﷺ.
RasoolAllah ﷺ received Wahi that he should emigrate. He asked Jibreel Alayhis Salaam with whom should he emigrate? Jibreel Alsyhis Salaam replied, with Abu Bakr Siddeeq. (Mawahibul Ladunya, Imam Qastalani)
RasoolAllah ﷺ and Sayyidunah Abu Bakr Radi Allahu Anhu silently left Makkah. This news soon reached the Makkans who angrily set a bounty for RasoolAllah ﷺ.
RasoolAllah ﷺ went to a cave in Mt Thawr, where he and Sayyiduna Abu Bakr Radi Allahu Anhu waited for at least 3 days. Meanwhile the Makkans combed the entire area including the cave at Mt Thawr. In Juz/Para 10, in a verse rich in meaning, Allah ﷻ describes the events at the cave:
Allah had helped him (the Beloved Prophet ﷺ) (and) the second of the two when those who did not believe caused him to leave when both were in the cave as he said to his companion do not be anxious certainly Allah is with us then Allah sent down His comfort to him and He supported him with an army (that) you did not see and He put down the words of those who did not believe and the Speech of Allah it is the loftiest and Allah is Azeez Hakeem. (Surah Tauba v 40)
As the Makkans neared the cave, Sayyiduna Abu Bakr Radi Allahu Anhu became concerned, and the Quran records:
he (the Beloved Prophet ﷺ) said to his companion do not be anxious certainly Allah is with us then Allah sent down His Comfort to him (Surah Tauba v 40)
Here the Quran refers to Sayyiduna Abu Bakr Radi Allahu Anhu as RasoolAllah’s ﷺ Saahibi – Companion. Thus the Eternal Quran clearly affirms that Sayyiduna Abu Bakr Radi Allahu Anhu is the companion (Sahabi) of RasoolAllah ﷺ. Thus to accept Sayyiduna Abu Bakr Radi Allahu Anhu as a Sahabi is established from the Quran and not accepting him as a Sahabi would be going against the Quran. (Tafseer Imam Raazi, Tafseer Imam Qurtubi)
Imam Qastalani draws an important lesson from RasoolAllah ﷺ saying: certainly Allah is with us (Surah Tauba v 40)
Comparatively when Firoun was chasing after Nabi Musa ﷺ and his followers, they reached the sea/river. His followers feared that Firoun would capture them. The Quran records that Nabi Musa Alayhis Salaam replied: My Lord is with me (Surah Shuaraa v 62)
The difference is that RasoolAllah ﷺ had provided Sayyiduna Abu Bakr Radi Allahu Anhu with his Noor through which he was able to witness the secret of the Presence of Allah ﷻ. Consequently Sayyiduna Abu Bakr Radi Allahu Anhu received the secret of Sakeenah (comfort / peace of mind) for the Quran describes: then Allah sent down His Comfort to him (Surah Tauba v 40) (Mawahibul Ladunya)
Imam Raazi explains further that this is a great distinction for Sayyiduna Abu Bakr Radi Allahu Anhu for RasoolAllah ﷺ is partnering him in his being in the Presence of Allah ﷻ. (Tafseer Kabeer)
In this verse of Surah Tauba, Allah ﷻ also refers to Sayyiduna Abu Bakr t as ( ثَانِىَ اثنين ). This phrase has two meanings, both of which are applicable:
The second of the two, and
The next of the two.
In terms of the first meaning, the second of the two, Imam Raazi ﷺ indicates that it means that after RasoolAllah ﷺ, Sayyiduna Abu Bakr ﷺ was the second Muslim. (Tafseer Kabeer)
We can understand the second meaning, ‘the next of two’ from the Sahaba Radi Allahu Anhum themselves. After the angel of death had visited RasoolAllah ﷺ, the Sahaba were discussing who should be the next ruler. Sayyiduna Umar Radi Allahu Anhu quoted this very verse and indicated that Allah ﷻ has named Sayyiduna Abu Bakr ( ثَانِىَ اثنين ) the Next of the Two, thus he is the next to rule Muslims. (Tirmidhi)
So we find that in this verse of Surah Tauba, Allah Ta’aala has bestowed Sayyiduna Abu Bakr Radi Allahu Anhu with several blessings, including being a Sahabi (Companion) of RasoolAllah ﷺ, being granted the Comfort of Allah ﷻ and being ( ثَانِىَ اثنين ) the second Muslim and the next leader after RasoolAllah ﷺ.