Mishkat al-Masabih #1507
Funerals
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَعَنْ أَنَسٍ قَالَ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لَا يَعُودُ مَرِيضًا إِلَّا بَعْدَ ثَلَاثٍ. رَوَاهُ ابْنُ مَاجَهْ وَالْبَيْهَقِيُّ فِي شُعَبِ الْإِيمَانِ
Anas said the Prophet was not accustomed to visit an invalid till he had been ill for three days.
Ibn Majah and Baihaqi, in Shu'ab al- iman, transmitted it.
About Mishkat al-Masabih
This hadith is #1507 from Mishkat al-Masabih, in the Book of Funerals.