It is narrated from al-Harith bin Qays (may Allah have mercy on him) that he said: "When you are engaged in one of the affairs of this world, complete it swiftly and briskly; and when you are engaged in one of the affairs of the Hereafter, take your time over it and keep composure as much as you can; and when you intend some good deed, do not delay it; and if you are praying and Satan comes to you and says, 'You are showing off,' then make that prayer even longer."
Musaddad recorded it in his Musnad, and all the narrators of its chain are thiqat (trustworthy) and reliable.
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / صفة الصلاة / 495
Hadith Gradingمحدثین:إسناده صحيح
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :إسنادہ صحیح » «أخرجه النسائي فى «الكبریٰ» برقم: 11856، وأورده ابن حجر فى "المطالب العالية"، 495، وأخرجه ابن أبى شيبة فى «مصنفه» برقم: 8443، 36076، 36770»