Spiritual Retreat: Sitting Under a Shade While Outside the Mosque
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1756
A muʿtakif must not stay outside the mosque for longer than is necessary; and while he is outside, if possible, he must not sit under a shade. However, if he cannot help sitting under a shade in order to attend to the matter in question, there is no problem. And based on obligatory precaution, after he has attended to the matter, he must not sit at all unless it is necessary for him to do so.
-Ayatullah Sistani, Practical Laws of Islam, Spiritual Retreat (Iʿtikāf)