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

أَعْتَمَ رسول الله صلى الله عليه وسلم ليلة من الليالي بصلاة العشاء، وهي التي تُدْعَى العَتَمَةَ، فلم يخرج رسول الله صلى الله عليه وسلم حتى قال عمر بن الخطاب: نام النساء والصبيان [وفي رواية: حتى ذهب عامَّة الليل]، فخرج رسول الله صلى الله عليه وسلم ، فقال لأهل المسجد حين خرج عليهم: «ما ينتظرها أحد من أهل الأرض غَيْرُكُم»، وفي رواية: «إنه لوقتها لولا أن أشق على أمتي». وفي رواية: «لولا أن يُشَقَّ على أمتي»، وذلك قبل أن يفشوَ الإسلام في الناس. قال ابن شهاب: وذُكِر لي: أن رسول الله صلى الله عليه وسلم ، قال: وما كان لكم أَن تَنْزُرُوا رسول الله صلى الله عليه وسلم على الصلاة، وذاكَ حِين صاح عمر بن الخطاب.
[صحيح] - [رواه مسلم]

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.”
[Authentic hadith] - [Narrated by Muslim]

Explanation

In this Hadīth, the Messenger of Allah (may Allah’s peace and blessings be upon him) pointed out the best time for performing the ‘Ishā' Prayer, which is the last part of the first third of the night. However, he did not perform it constantly at this time out of mercy towards his Ummah and fearing that he would thus make things too difficult for them.