Allah recounts some of the events of the Prophet's migration from Makkah to al-Madinah in surah al-Tawbah [9] when He says:
[إِذْ هُمَا فِي الْغَارِ إِذْ يَقُولُ لِصَاحِبِهِ لَا تَحْزَنْ إِنَّ اللَّهَ مَعَنَا]
... when the two of them were in the cave and he said to his companion, "Do not worry; Allah is with us." [40]
This ayah indicates some of the virtues of Abu Bakr al-Siddiq.
Allah confirmed the truth of the Prophet's statement to Abu Bakr and described him in His Book as a "companion".
Some of the scholars have said: If anyone denies that 'Umar, 'Uthman or any of the sahabah were companions of Allah's Messenger, then they are a liar and an innovator. But if anyone denies that Abu Bakr was a companion of Allah's Messenger, then they are a disbeliever for they have rejected an explicit statement of the Qur'an.
[Jaami' li-Ahkam al-Qur'an 8/77]