The Messenger of Allah ﷺ said, "When the prayer has begun, do not come to it running. Rather, come walking with calmness and dignity. Pray whatever you catch with the congregation, and complete whatever you missed."
حدثنا عبد الرزاق عن معمر قال الزهري وقد أخبرني سعيد بن المسيب
أن رسول الله ﷺ قال إذا أقيمت الصلاة فلا تأتوها تسعون ولكن ائتوها وأنتم تمشون وعليكم السكينة فما أدركتم فصلوا وما فاتكم فأتموا
Its isnad is sahih, and it is a repetition of 7229, 7249, 7251 with similar wording, from various routes, from Abu Hurairah.His saying, "and upon you calmness," may be read with the accusative case, functioning as an exhortation, or with the nominative case, on the basis that the clause stands as a circumstantial statement. Both readings are established in the Yunini manuscript of Bukhari [1:129, and 2:7–8, of the Sultaniyyah edition]. See also Fath al-Bari 2:97–98, and our commentary on Tirmidhi, numbers 327–329, [vol. 2, pp. 148–150].