Sahih al-Bukhari, Volume 5, Book 59, Hadith 720 (USC-MSA) ⓘ
Narrated `Aisha: When the Prophet fell ill in his fatal illness, he started saying, "With the highest companion."
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 " .
also referenced under: Book 64, Hadith 457 (In-Book) & Hadith 4436 (DarusSalam)