Adapted from the book, Min Akhlaaq Salaf of Shaykh Ahmad Fareed; rendered into English by Ishaaq bn AbdirRaheem
Allâh the Great said:
“And whatsoever the Messenger (Muhammad) gives you, take it, and whatsoever he forbids you, abstain (from it), and fear Allâh. Verily, Allâh is Severe in punishment.” [al-Hashr: 7].
He, the Most Great, also said:
“And let those who oppose the Messenger's (Muhammad) commandment (i.e. his Sunnah legal ways, orders, acts of worship, statements, etc.) (among the sects) beware, lest some Fitnah (disbelief, trials, afflictions, earthquakes, killing, overpowered by a tyrant, etc.) befall them or a painful torment be inflicted on them.” [Nuur: 63].
The Prophet (r) said:
“إياكم ومحدثات الأمور فإن شر الأمور محدثاتها وإن كل محدثة بدعة "
‘Beware of the newly introduced affairs (in the Dîn), for every innovation (in the Dîn) is misguidance.’ 
He (r) also said:
"من عمل عملا ليس عليه أمرنا فهو رد"
‘Whoever does an act which does not have our command then such is to be rejected.’
The Salaf thus used to enjoin people on restricting themselves to the Qur’ân and Sunnah, and shunning innovations; the Salaf used to be hard on this, such that many a times would Umar bn Khattâb (ru) think on an act being done, and when he would be told that such had not been sanctioned by the Prophet he would refrain from the act.
One of the Salaf (rh) had said:
‘Verily the path of (these) people (that is, the Salaf) has been hinged on the Qur’ân and the Sunnah the like of gold and pearl, this is so because in their every action and inaction was a good intention in accordance to the Islamic standard, none would know this except he who is deep in the Islamic learning.’
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 Ahmad reported it (4/126, 127), and Abu Dawud (12/359, 360) in the Book of Sunnah, and Tirmidhî (10/144) in the Book of Knowledge, and he said: ‘It is Hasan Sahih’; and Ibn Mâjah (43) in the Muqaddimah; and Ad-Dârimee (1/44, 45) in the Book of Following the Sunnah; and Al-Baghawi in Shar’hu Sunnah (1/205), and he said: ‘The hadith is Hasan.’ And Al-Albâni authenticated it in Adh-Dhilâlu-Jannah.
 Muslim reported it in the Book of Judgements (12/16); and Al-Bukhâri reported what is equal to it in meaning, in the Book of Settlement (5/301).
The fact that the deed will be rejected here implies that the deed is null and void, and of no value.