The Prophet ﷺ forbade sitting between sunlight and shade, saying, "It is the seat of Satan."
حدثنا بهز وعفان قالا حدثنا همام قال عفان في حديثه حدثنا قتادة عن كثير عن أبي عياض
أن النبي ﷺ نهى أن يجلس بين الضح والظل وقال مجلس الشيطان
Its isnad is sahih. Kathir is Ibn Abu Kathir Basri, the freed slave of Abdul-Rahman bin Samura, and he is reliable. Abu Iyad is Amr bin Aswad Ansi, a reliable Mukhadram from the senior generation of the Followers, who did not meet the Prophet ﷺ. The anonymity of the companion here does not cause harm, because the narrator from him is reliable and did not narrate in a disconnected form. Haythami likewise graded it sahih, 8/60. Bayhaqi narrated it with its wording and its isnad in Sunan al-Kubra 3/236-237, and Abu Dawud narrated it with its meaning, 4/257, number 4821, in the chapter on manners, sitting between the shade and the sun, as did Humaydi, 2/482, number 1138. Adh-Dhah is the place where the sun falls.