Sincerity (ikhlaas) towards Allaah means that the person desires by his worship closeness to Allaah the Exalted and reaching His Abode of Nobility (i.e. Paradise), such that the servant must be sincere towards Allaah the Exalted in his intended purpose. 
He must be sincere towards Allaah the Exalted in his love. He must be sincere towards Allaah in his exaltation. He must be sincere to Allaah the Exalted outwardly and inwardly. 
He does not seek anything with his worship, except for the Face of Allaah the Exalted and reaching His Abode of Nobility.

 
Imaam Muhammad Ibn Saalih al-'Uthaymeen (d.1421H) - rahimahullaah.