The Prophet ﷺ entered the mosque while Abu Israil was praying. It was said to the Prophet ﷺ, "Messenger of Allah, this man has vowed not to sit, not to speak to people, not to seek shade, and he intends to fast."The Prophet ﷺ said, "Let him sit, let him speak to people, let him seek shade, and let him fast."
حدثنا عبد الرزاق حدثنا ابن جريج ومحمد بن بكر قال أخبرني ابن جريج قال أخبرني ابن طاوس عن أبيه
دخل النبي ﷺ المسجد وأبو إسرائيل يصلي فقيل للنبي ﷺ هو ذا يا رسول الله لا يقعد ولا يكلم الناس ولا يستظل وهو يريد الصيام فقال النبي ﷺ ليقعد وليكلم الناس وليستظل وليصم
No references are available.
Its isnad is sahih, its narrators are trustworthy and well known. Haythami also graded the hadith sahih 4/188, and attributed it to Ahmad and Tabarani.