The author of Zād al-Mustani’ (rahimahullāh) stated: “It is not permissible to delay making up the fasts until after the following Ramadān without an excuse. If he does that, then he must, alongside making up the fast, also feed a poor person for each day.” (Sharhul-Mumti’, 6/446, 450)
Shaikh Ibn ‘Uthaimīn explained: The qadā (making up the missed fasts) is an obligation because it is a debt he is responsible for that he has not fulfilled, so he must fulfil it. As for feeding the poor for each day, then that is because he delayed making up the fast within the given time (i.e. before the following Ramadan). Therefore, he feeds a poor person for each day that he missed ―and this is the madhhab of Imam Ahmād and Imām Mālik (rahimahumallāh).
So, if we assume he missed six days of fasts, then alongside making up the six fasts (which he delayed beyond the following Ramadan), he also feeds six poor people. This has been reported in a narration ascribed to the Prophet (salallāhu ‘alaihi wasallam) which states: “He commanded feeding the poor alongside qadā for the one who delayed making up his fasts until after the following Ramadan.”  However, this narration is very weak and cannot be used to establish proof or to discharge the responsibility.
It is also reported from Ibn ‘Abbās and Abu Hurayrah (radiyallahu ‘anhum) that they said: “He must feed a poor person [for each day].” However, that which has been mentioned from them is not a proof because proof cannot be established except by the Book and Sunnah ―and as for the sayings of the Sahābah, then its proof is questioned if it opposes what is apparent from the Qur’an. So, here the obligation of feeding a poor person for each day that was missed opposes what is apparent from the Qur’an because Allah did not obligate except making up the missed days. He did not obligate more than that. So based upon that, we do not obligate upon the servants of Allah that which Allah did not obligate upon them ―except with a proof that frees one of the responsibility ―and it is possible to understand that which has been narrated from Ibn ‘Abbās and Abu Hurayrah (radiyallahu ‘anhum) from the aspect of recommendation and not from the aspect of obligation. So what is correct in this affair is that one is obligated to do no more than make up the fasts he has missed ―and that he is sinful for delaying in making them up. 
Source: See Sharhul-Mumti’, 6/450 of Al-‘Allāmah, Ash-Shaikh, Muhammad bin Sālih Al- ‘Uthaimīn (rahimahullāh) and his Majmū’ Fatāwā, 19/373-395).
 Reported by Ad-Dāruqutni in the book of fasting (1/252), Al-Bayhaqi 4/253 from Abu Hurayrah (radiyallāhu ‘anhu). About some of the narrators Ad-Dāruqutni said: “Ibrahim bin Nāfi’ and Ibn Wajeeh are both weak.” Al-Bayhaqi said: “It is nothing [because] Ibrahim and ‘Umar are abandoned.”
 The narration of Ibn ‘Abbās was reported by Al-Bayhaqi (4/253) and An-Nawawi said in Al-Majmoo’ (6/364), “Its isnād is saheeh.” The narration of Abu Hurayrah is reported by Ad-Dāruqutni (2/196) who said: “Its isnād is saheeh mawqoof.”
 The madhhab of the Hanifies is that a person does not pay a penalty (kaffārah) of feeding the poor. See Al-Hidāyah (2/354), Dār al-Mukhtār (2/423).