This is the best time to perform it, had it not been difficult for my Ummah

The Messenger of Allah (may Allah’s peace and blessings be upon him) delayed the ‘Ishā' prayer, which is called the ‘Atamah, one night. He did not come out till ‘Umar ibn al-Khattāb said: "The women and children had gone to sleep." [In another narration: Until most of the night had elapsed]. The Messenger of Allah (may Allah’s peace and blessings be upon him) came out and said to the people in the mosque: "None among the inhabitants of earth except you are waiting for it." [In another narration: This is the best time to perform it, had it not been difficult for my Ummah.] This happened before the spread of Islam.
Ibn Shihāb said: "It was reported to me that the Messenger of Allah (may Allah’s peace and blessings be upon him) said: ‘It was not appropriate for you to press the Messenger of Allah to come to prayer’, alluding to ‘Umar ibn al-Khattāb when he gave his loud notification.”