1) Aboo Hurayrah رضى الله عنه said: Abool-Qaasim صلى الله عليه وسلم said: “If one of you was sitting in the sunlight”, and Makhlad said: “in the shadow, so the shade that is over him moves away from him, and part of him is in the sun and part (of him) is in the shade, then let him get up (and move).” 
2) And in another narration: He prohibited that one sit between the sunlight and the shade, and he said: “It is the sitting of shaytaan.” 
3) Qays bin Abee Haazim narrated from his father that he said: The Prophet صلى الله عليه وسلم saw me and I was sitting in the sun, sohe said: “Move to the shade.”
In the wording of Aboo Daawood (#4822), Qays reported that his father narrated that he came and the Messenger of Allaah صلى الله عليه وسلم was giving a khutbah, so he stood in the sun, so he ordered him to move to the shade. 
 Aboo Daawood (#4821), al-Humaydee in al-Musnad (1138), Ahmad (2/383). Shaykh al-Albaanee declared it saheeh. See As-Saheehah (#837).
 Collected by Ahmad (3/413). Shaykh al-Albaanee said: “And this isnaad is saheeh, it’s men are thiqaat (trustworthy narrators), men of the Shaykhayn (al-Bukhaaree and Muslim), with the exception of Katheer, and he is Ibn Abee Katheer al-Basree as
al-‘Ajalee and Ibn Hibbaan have said, and a group from the thiqaat narrated from him. See As-Saheehah (#838).
 Declared saheeh by Shaykh al-Albaanee. See As-Saheehah (#833)
In another riwayah,
If the place where one is sitting falls between the sun and the shade, he should change his place, because the Prophet Muhammed Sallallahu alaihi wasallam said:
“If one of you is (sitting) in shade that diminishes in size, resulting in part of his body remaining in the sun and the rest under shade, he should get up and move.
(Abu Dawood, no. 4821; Saheeh al-Jaami, no. 748)”
The reason for this is that this is the position favoured by the Shaytaan. The evidence is the Prophet‘s prohibition of sitting partially in the sun and partially in shade. He Sallallahu alaihi wasallam said:
This is the place where the Shaytaan sits.
(Ahmad, 3/413; Saheeh al-Jaami, No. 6823)
…May Allah give us a better understanding & guide us to accept the truth