Sahih al-Bukhari 4436
Sahih al-Bukhari · Book 64: Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi) · The sickness of the Prophet (saws) and his death
Narrated `Aisha:
حَدَّثَنَا مُسْلِمٌ ، حَدَّثَنَا شُعْبَةُ ، عَنْ سَعْدٍ ، عَنْ عُرْوَةَ ، عَنْ عَائِشَةَ ، قَالَتْ لَمَّا مَرِضَ النَّبِيُّ صلى الله عليه وسلم الْمَرَضَ الَّذِي مَاتَ فِيهِ جَعَلَ يَقُولُ " فِي الرَّفِيقِ الأَعْلَى " .
When the Prophet (ﷺ) fell ill in his fatal illness, he started saying, "With the highest companion."