Mishkat al-Masabih 5 Funerals كتاب الجنائز 65

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.