«إذا أتيت الصلاة فأتها بوقار وسكينة فصل ما أدركت واقض ما فاتك» .
(صحيح) … [طس] عن سعد. الصحيحة 1198)
It is reported that the Messenger of Allah (peace and blessings of Allaah be upon him) said:
“If you go to prayer, then go to it with dignity and calmness. Do (with the imam that part of the salah that) you found, and make up for what you missed. ”
This hadith was narrated by at-Tabarani in Mu’jam al-Awsat 1357 from the words of Sa’d.
Sheikh al-Albani called the hadith authentic. See Saheeh al-jami ‘as-saghir 272.