Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.042.
Section : Before performing ghusl.
It is preferred to perform ablution before ghusl regardless of whether that particular ghusl was an obligatory or a preferred act. Said ‘Aishah, “When the Messenger of Allah, upon whom be peace, performed post-sex ghusl, he would begin by washing his hands and then pour water from his right hand to his left and wash his private parts. He would then perform ablution..” (Related by “the group.”)