AbulQasim ﷺ said: When one of you is in the sun (Shams)--Makhlad's version has "fay'"--and the shadow withdraws from him so that he is partly in sun and partly in shade, he should get up.  

أَبُو الْقَاسِمِ ﷺ إِذَا كَانَ أَحَدُكُمْ فِي الشَّمْسِ وَقَالَ مَخْلَدٌ فِي الْفَىْءِ فَقَلَصَ عَنْهُ الظِّلُّ وَصَارَ بَعْضُهُ فِي الشَّمْسِ وَبَعْضُهُ فِي الظِّلِّ فَلْيَقُمْ  

Qais quoted his father as saying that he (his father) came when the Messenger of Allah ﷺ was addressing. He stood in the sun. He ordered him (to shift) and he shifted to the shade.  

أَنَّهُ جَاءَ وَرَسُولُ اللَّهِ ﷺ يَخْطُبُ فَقَامَ فِي الشَّمْسِ فَأَمَرَ بِهِ فَحُوِّلَ إِلَى الظِّلِّ