Yahya said that Malik was asked about the hadith attributed to the Prophet (ﷺ) "Whoever cannot find a waist wrapper should wear trousers," and he said, "I have never heard this, and I do not think that some one who is in ihram can wear trousers, because among the things which the Prophet (ﷺ) forbade some one in ihram to wear were trousers, and he did not make any exception for them although he did make an exception for leather socks."
20:4 Wearing Clothes when in Ihram
Arabic : Book 20, Hadith 715