It is better that when one has the option to, he says in rukūʿ:...although saying any dhikr suffices; and based on obligatory precaution, [the other dhikr] must be of this length. However, if time is short or one is compelled, then saying subḥānal lāh once suffices. Someone who cannot say subḥāna rabbiyal ʿaẓīmi wa biḥamdih properly must say another dhikr, such as subḥānal lāh, three times.