All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
It is desirable for a praying person to say, “Subhaana Rabbiyy Al-'Atheem (how perfect my Lord is, the Great)” three times when in the bowing position in prayer, but it is permissible to say it only once. Ibn Qudaamah said in Al-Mughni, “The praying person should say Subhaana Rabbiyy Al-'Atheem (how perfect my Lord is, the Great) three times, but it is acceptable if he says it only once.” It is also desirable for him to say when bowing and prostrating: “Suboohun Quddoos Rabbul-Malaa'ikati war-Rooh (how Perfect and Holy He is, Lord of the angels and the Rooh (Angel Gabriel).” Imaam Muslim reported that ‘Aa’ishah narrated that the Prophet, sallallaahu ‘alayhi wa sallam, would say: “Suboohun Quddoos Rabbul-Malaa'ikati war-Rooh (how Perfect and Holy He is, Lord of the angels and the Rooh (Angel Gabriel),” when in a bowing position and in prostration.
Imaam Ahmad added that the Prophet, sallallaahu ‘alayhi wa sallam, would say this three times, although the narrator of this narration had doubts about this addition in the wording in the narration of Imaam Ahmad. Thus it is better if the praying person says it three times, but if he says it only once, then that is also correct and the matter is easy, and all perfect praise be to Allaah.
Allaah Knows best.