The word(s) "observe" appears 13 time(s) in 12 verse(s) in Quran in Mohsin Khan translation.

(11) Then, We sent after them Our Messengers, and We sent 'Îsa (Jesus) - son of Maryam (Mary), and gave him the Injeel (Gospel). And We ordained in the hearts of those who followed him compassion and mercy. But the monasticism which they invented for themselves, We did not prescribe for them, but (they sought it) only to please Allah therewith, but they did not observe it with the right observance. So We gave those among them who believed their (due) reward; but many of them are Fasiqun (rebellious, disobedient to Allah).

(12) Have you (O Muhammad صلى الله عليه وسلم) not observed the hypocrites who say to their friends among the people of the Scripture who disbelieve: "(By Allah) If you are expelled, we (too) indeed will go out with you, and we shall never obey any one against you; and if you are attacked (in fight), we shall indeed help you." But Allah is Witness that they verily are liars.