Bhagavad Gita: Chapter 5, Verse 4

BG 5.4: Only the ignorant speak of sānkhya (renunciation of actions, or karm sanyās) and karm yog (work in devotion) as different. Those who are truly learned say that by applying ourselves to any one of these paths, we can achieve the results of both.

Here, Shree Krishna uses the word sānkhya to refer to karm sanyās, or the renunciation of actions with the cultivation of knowledge.  It is important to understand here that renunciation is of two kinds:  phalgu vairāgya and yukt vairāgya.  Phalgu vairāgya is where people look upon the world as cumbersome, and renounce it with the desire of getting rid of responsibilities and hardships.  Such phalgu vairāgya is an escapist attitude and is unstable.  The renunciation of such persons is motivated by the desire to run away from difficulties.  When such persons encounter difficulties on the spiritual path, they become detached from there as well, and desire to run back to worldly life.  Yukt vairāgya is where people see the whole world as the energy of God.  They do not see what they possess as belonging to them, and do not wish to enjoy it for themselves.  Instead, they are motivated by the desire to serve God with whatever He has given to them.  Yukt vairāgya is stable and undeterred by difficulties.

The karm yogis, while conducting their daily duties externally, develop the sentiment of yukt vairāgya, or stable renunciation.  They see themselves as the servants and God as the enjoyer, and hence they become fixed in the consciousness of doing everything for His pleasure.  Thus, their internal state becomes the same as that of the karm sanyāsīs, who are completely absorbed in divine consciousness.  Externally, they may appear to be worldly people, but internally they are no less than sanyāsīs. 

The Puranas and Itihās relate the examples of great kings in Indian history, who, though externally discharging their kingly duties with diligence and living in royal opulence, were mentally completely absorbed in God-consciousness.  Prahlad, Dhruv, Ambarish, Prithu, Vibheeshan, Yudhishthir, etc. were all such exemplary karm yogis.  The Shreemad Bhagavatam states: 

gṛihītvāpīndriyair arthān yo na dveṣhṭi na hṛiṣhyati

viṣhṇor māyām idaṁ paśhyan sa vai bhāgavatottamaḥ   (11.2.48)

“One who accepts the objects of the senses, neither yearning for them nor running away from them, in the divine consciousness that everything is the energy of God and is to be used in His service, such a person is the highest devotee.”  Thus, the truly learned see no difference between karm-yog and karm sanyās.  By following one of them, the results of both are achieved.