Accompanying the Prophet
Translated
by
Abbas Abu Yahya
Ibn al-Qayyim rahimahullaah said about the saying of Allaah Ta’ala:
{إِذْ يَقُولُ لِصَاحِبِهِ لَا تَحْزَنْ إِنَّ اللَّهَ مَعَنَا}
<<when the Prophet and Abu Bakr were in the cave, and the Prophet said to his companion (Abu Bakr): “Do not be sad (or afraid), surely Allaah is with us.”>>
[At-Tawbah – 40]
From the most correct indication, is what is indicated in this Ayaah:

[Badai Tafseer 2/10]
Arabic Reference
صحب الرسول صلى الله عليه وعلى آله وسلم
قال ابن القيم رحمه الله تعالىٰ:
قَوله تَعالىٰ: {إِذْ يَقُولُ لِصَاحِبِهِ لَا تَحْزَنْ إِنَّ اللَّهَ مَعَنَا} فمن أصحّ الإشارات؛ إشارة هذه الآية وهي: أنّ من صحب الرسول صلى الله عليه وعلى آله وسلم، وما جاء به بقلبه وعمله، وإن لم يصحبه ببدنه، فإنّ الله معه.
[بدائع التفسير(٢/ ١٠)]
Also see @