Abu Bakrah (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) said: "Shall I inform you of the gravest of the major sins?" He repeated this three times, and then said: "Associating partners with Allah, mistreatment of parents." He was reclining, and then sat up and said: "And indeed the false statement and the false testimony." He kept repeating this so many times that we wished he should be quiet.
عن أبي بَكْرَةَ- رضي الله عنه - عن النبي -صلى الله عليه وسلم- أنه قال: «أَلا أُنَبِّئُكم بِأَكْبَرِ الْكَبَائِر؟»- ثَلاثا- قُلْنَا: بَلى يا رسول الله، قَالَ: «الإِشْرَاكُ بِالله وَعُقُوقُ الوالدين، وكان مُتَّكِئاً فَجَلس، وَقَال: ألا وَقَوْلُ الزور، وَشهَادَةُ الزُّور»، فَما زال يُكَرِّرُها حتى قُلنَا: لَيْتَه سَكَت.
شرح الحديث :
The Messenger of Allah (may Allah's peace and blessings be upon him) asked his Companions if they would like him to tell them about the gravest of all the major sins and then mentioned the following: 1. Associating others with Allah: This is a violation to the divine status of Allah and is usurping His right of divinity and undeservedly assigning it to one of the weak creation. 2. Maltreatment of parents: This is a grave sin because it repays kindness with harm to the closest relatives. 3. False testimony: This is applicable to any false statement or lie intended to steal someone's rights by taking his money, violating his honor, or anything to this effect.