When the sick have renounced all hope of life

أللّهُـمَّ اغْفِـرْ لي وَارْحَمْـني وَأَلْحِقْـني بِالرَّفـيقِ الأّعْلـى

O Allah, forgive me and have mercy upon me, and join me with the highest companions (in Paradise).

Al-Bukhari7/10, Muslim 4/1893.

جعل النبي صلى الله عليه وسلم عند موته يدخل يديه في الماء فيمسح بهما وجههُ ويقول : لا إله إلا الله إن للموت لسكرات

There is none worthy of worship but Allah, surely death has agonies.

Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 8/144. The Hadith also mention him using the Siivak (tooth stick).

لا إلهَ إلاّ اللّهُ وَاللّهُ أَكْبَـر، لا إلهَ إلاّ اللّهُ وحْـدَهُ، لا إلهَ إلاّ اللّهُ وحْـدَهُ لا شَريكَ لهُ، لا إلهَ إلاّ اللّهُ لهُ المُلكُ ولهُ الحَمْد، لا إلهَ إلاّ اللّهُ وَلا حَـوْلَ وَلا قُـوَّةَ إِلاّ بِالله

There is none worthy of worship but Allah, Allah is the Most Great. None has the right to be worshipped but Allah alone. None has the right to be worshipped but Allah alone, Who has no partner. There is none worthy of worship but Allah, His is the dominion and His is the praise. There is none worthy of worship but Allah, there is no power and no might but by Allah.

At-Tirmidhi and Ibn Majah. See also Al-Albani, Sahih At-Tirmidhi 3/152 and Sahih Ibn Majah 2/317.