In an authentic hadith, Imam al-Sadiq (as) explains of the importance of having an attentive heart/mind during prayers, sincerely thinking about your Lord in submission. Often we may become distracted and think about worldly affairs during our prayers – which may limit how much of our prayers is spiritually accepted by Allah (swt). Of course, since Allah (swt) knows the weakness of our minds/hearts, he has gifted us the opportunity to make up for that in the optional prayers (Nawafil).
عن أبيه، عن سعد بن عبد الله، عن أيوب بن نوح، عن ابن أبي عمير، عن هشام بن سالم، عن محمد بن مسلم قال: قال أبو عبد الله عليه السلام إن العبد لترفع له من صلاته نصفها أو ثلثها أو ربعها أو خمسها وما يرفع له إلا ما أقبل عليه منها بقلبه، وإنما أمرنا بالنوافل ليتم لهم بها ما نقصوا من الفريضة
‘Abu Abdullah (as) said: ‘For the servant (of Allah), either half of his prayer is raised up, or a third of it, or a quarter of it, or a fifth of it. And it is not Raised for him except what is acceptable to him, from it, in his heart. However, the optional prayer (al-Nawafil) is commanded so that it may complete what is deficient from the obligatory prayer’.
Sanad: My (al-Sadooq) father said, Sa’ad bin Abdullah narrated to us, from Ayoub bin Nuh, from ibn Abu `Umeyr, from Hisham bin Salim, from Muhammad bin Muslim who said the above hadith
Reference: `ilal al-Shara’i`, Volume 2, Chapter 24, Hadith #3 (See First Image below); Bihar al-Anwar, Volume 81, Page 238, Hadith #18 (link)
Grading: al-Mohsini says it has a Reliable (Mu`tabar) chain [Mashra`at Bihar al-Anwar, Volume 2, Page 419] (See Second Image below)