Nafi' (may Allah have mercy on him) says: Sayyiduna Ibn Umar (may Allah be pleased with them both) saw a man riding his camel who had made a shade between himself and the sun while he was in the state of ihram, and he said: Stay out in the sun for the One for whom you have entered ihram.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9192
Ata' (may Allah have mercy on him) says that he saw Abdullah bin Abi Rabi'ah (may Allah be pleased with him), who had placed a piece of wood across the middle of his she-camel and put a cloth over it, taking shade with it from the sun while he was in the state of ihram. Ibn Umar (may Allah be pleased with them both) met him and forbade him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9193
Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "There is no muhrim who stays in the sunshine until the sun sets, except that the sun sets taking his sins with it, until he becomes as he was on the day his mother gave birth to him."
This chain is weak; the report before it is mawquf (stopping at the Companion), and the hadith of Umm al-Husayn is a sahih (authentic) hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9194