The Messenger of Allah ﷺ said, "When the call to prayer is made, come to it walking calmly. Whatever part of the prayer you catch, pray it, and whatever you have missed, make it up."
وقال رسول الله ﷺ إذا نودي بالصلاة فأتوها وأنتم تمشون عليكم بالسكينة فما أدركتم فصلوا وما فاتكم فاقضوا
This is a sahih hadith, narrated by Muslim in volume 2, page 100, and by Bukhari in volume 1, page 164, Shaab edition. Abu Dawud narrated it in the Book of Prayer from Abu Walid and from Ahmad bin Salih. Nasai narrated it in the Book of Prayer from Abdullah bin Muhammad with the wording, "When the prayer is called, do not come to it running." Ibn Majah narrated it, and in his version it is from Abu Marwan Uthmani. Malik narrated it in Muwatta in the chapter on walking to prayer and the merit of mosques with the wording, "When the call to prayer is given, do not come to it while running, but come to it calmly. Whatever you catch of it, pray, and whatever you miss, complete afterward, for each of you is in prayer as long as he intends to go to the prayer."The meaning of thuwwiba is that the call was given. This hadith indicates that whoever catches the ruku has caught the rakat, without any requirement of reciting the Fatihah.