Adhering to the Sunnah is better than merely doing more good deeds
“Adhering to the Sunnah is better than merely doing more good deeds”.
Shaykh Muhammad ibn Saalih al-Uthaymeen rahimahullaah said:
The evidence for this is what Abu Dawud rahimahullaah has reported in his Sunan:
Two persons set out on a journey. Meanwhile the time of prayer came and they had no water. They performed Tayammum with clean earth and prayed.
Later on they found water within the time of the prayer.
One of them repeated the prayer and ablution but the other did not repeat. Then they came to the Messenger of Allah sallallaahu alayhi wa sallam and related the matter to him. Addressing to the one who did not repeat, he sallallaahu alayhi wa sallam said:
“You followed the Sunnah and your (first) prayer was enough for you.” He sallallaahu alayhi wa sallam said to the one who performed ablution and repeated:
“For you there is the double reward.”
[Sunan Abu Dawud (338) and Sunan al-Nasa’i (433) and graded as “Saheeh” by Shaikh al-Albaani]
So if a person says: “I want to earn the reward twice.”
We say to him: “If you know the Sunnah, then you will not have two rewards; rather that act will be treated as Bid’ah. As for the one who repeated the prayer and to whom the Prophet sallallaahu alayhi wa sallam said: “For you there is the double reward”, he did not know the Sunnah and did his Ijtihaad, so he received two rewards: i.e. for praying twice.”
And from this Hadeeth we learn a very important benefit and that is: Adhering to the Sunnah is better than doing more good deeds.
Example: Praying more optional prayers between the Adhaan and the Iqaamah of the Fajr prayer is not prescribed because the Prophet sallallaahu alayhi wa sallam did not do that (nor did he inform others to do it).
Similarly, if someone wants to prolong the optional prayer of two Rak’aat of Fajr by prolonging the recitation, the Rukoo and the Sujood because there is lot of time – between the Adhaan and the Iqaamah – we say to him: “You have opposed the Sunnah because the Prophet sallallaahu alayhi wa sallam used to pray this as two light Rak’aat.” 
Or if someone intends to pray four Rak’aat consecutively behind the Maqaam (of Ibraaheem) after the Tawaaf or if he intends to prolong the two Rak’aat behind the Maqaam after the Tawaaf, we say to him: “This is wrong, because the Prophet sallallaahu alayhi wa sallam used to pray them light and he never increased it more than two Rak’aat.” 
Taken from: Sharh al-Mumti` (1/406-407) of Shaykh al-Uthaymeen
 Narrated `Aaishah radiallaahu anha: “The Prophet sallallaahu alayhi wa sallam used to make the two Rak’aat before the Fajr prayer so light that I would wonder whether he recited Al-Fatihah (or not).” [Saheeh al-Bukhari (1171) and Saheeh Muslim (724)]
 As reported by Imam Muslim in his Saheeh from the Hadeeth of Jabir bin `Abdullah (2803) [and in it is mentioned that the Prophet sallallaahu alayhi wa sallam recited Surah al-Ikhlas and Surah al-Kafirun in these two Rak’aat], and also from the Ahadeeth of Ibn `Umar (2832, 2848, 2894).