It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "None of you should ever stand by a wronged person who is being killed, for when those present do not defend him, the curse descends upon them; and no one should ever stand by a wronged person who is being beaten, for when those present do not defend him, the curse descends upon them too."
This narration is from the book Musnad Ahmad bin Mani'; however, in its chain Husayn bin Qays al-Rahabi is a weak narrator.
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / كتاب الخلافة والإمارة / 2135
Hadith Gradingمحدثین:إسناده ضعيف جدًا
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :إسناده ضعيف جدًا. » «أخرجه المعجم الكبير للطبراني برقم: 11675»